This invention pertains to a leakage inspection method for a sealed container in which a liquid is liquid-tightly contained for inspecting whether a defect such as a pinhole or a crack causing a leakage of the liquid exists in the container and a leakage inspection apparatus for carrying out the leakage inspection method.
A liquid such as an injection and an eye lotion that is required to avoid contamination by bacteria and deterioration by oxidization etc. is liquid-tightly preserved in a container formed of glass or plastic materials. The container to contain this kind of liquid is not allowed to have leakage. To this end, in the manufacturing process of products such as a medicine which should be shipped in a sealing manner, it is required to inspect whether a leakage generating a defect such as a pinhole or a crack, etc exists in the container or not.
In order to inspect the leakage of the sealed container formed of an insulating material such as a glass, a plastic material or the like in which the liquid is contained, there is known a method of observing a discharge phenomenon generated when an inspection voltage of AC high voltage is applied across the container.
As electrodes are put onto two places (inspection parts) of an outer surface of the insulating container in which the electrolytic liquid is contained and the AC high voltage is applied across the electrodes as the inspection voltage, an AC current flows through an electrostatic capacitance between the electrodes and the liquid in the container and through the liquid in the container. In this case, as a crest value of the inspection voltage is set up at a proper value, if the defect which causes the leakage in the inspection part does not exist, a feeble current just flows, but if the defect such as the pinhole or the crack exist in the inspection part and the leakage arises, or if the leakage may possibly arise, an electric discharge arises in the defective portion, and the current which flows across the electrodes increases. Thus, in the case where the liquid in the container is an electrolyte, whether the defect that causes the leakage exists in the inspection part where the electrodes are put from the crest value of the current flowing through the electrodes can be detected.
In the prior art inspection method, the AC current flowing through the electrodes placed onto the container to be inspected and through the container and the contained liquid is detected by a current detection resistance inserted in series to the grounded electrode and the crest value of the thus detected AC current is compared with a threshold value to judge that the leakage causing the defect exists when the crest value of the AC current is higher than the threshold value.
In the case where the liquid in the container is a non-electrolyte having a high resistance, even though the AC high voltage as the inspection voltage is applied across the container, the electric discharge never occurs across the defective portion and a fore-discharge phenomenon is just generated. In this state, since the comparative value relation between the crest value of the current detected when the defect exists and the crest value of the current detected when the defect does not exist is unfixed and in addition thereto, since the difference between the crest value of the current detected when the defect does not exist and the crest value of the current detected when the defect exists cannot be made large, the prior art method in which the crest value of the AC current flowing when the inspection voltage is applied is compared with the threshold value cannot clearly judge the existence of the defect.
In the case where the liquid in the container is the electrolyte having the lower resistance, the existence of the defect can be detected by comparing the crest value of the detected AC current with the threshold value. However, in the case where the liquid in the container is the electrolyte, even though there is no defect in the container, the momentary discharge is generated due to conditions such as a circumference environment, an existence of containment of the surface of the container, etc. so that the pulse noise overlaps the detected current waveform, which possibly causes the misjudgment of the existence of the defect or the mistaking of a good container for an inferior container. Although it is considered that the threshold value for the judgment is made higher in order to avoid such a misjudgment, the ratio of the inferior articles relative to the good articles will undesirably get higher by using the higher threshold value.
On the other hand, if the threshold value for the judgment of the quality is set up at a lower value so that the inspection is performed for a safety, the rate in which the good articles are judged as the inferior articles gets higher, the yield of the product is reduced and the high cost is caused.
Furthermore, in the case where the high voltage is applied across the insulating container formed of a glass, a plastic material etc., the quality of the container is possibly deteriorated and therefore the inspection voltage is desirably set up at a value as low as possible.
Accordingly, it is a principal object of the invention to provide a leakage inspection method for a sealed container adapted to detect an existence of a defect such as a pinhole or a crack with a high accuracy even though a liquid is either of an electrolyte or a non-electrolyte and to provide a leakage inspection apparatus used for carrying out such a leakage inspection method.
It is another object of the invention to provide a leakage inspection method for a sealed container adapted to perform an inspection even though a voltage used for the inspection is set up at a lower value with a high accuracy and to provide a leakage inspection apparatus used for carrying out such a leakage inspection method.
The present invention relates to a leakage inspection method having as an object of an inspection an insulating container in which a liquid is sealed and comprising the steps of applying an inspection voltage of high AC voltage across the container; detecting an AC current flowing through the container and the contained liquid and judging from a detection signal of the AC current whether there exists a defect in the container causing a leakage of the liquid.
In the invention, there is provided an A/D converter for converting an instantaneous value of the detection signal of the AC current into a digital value and there is arithmetically operated a total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current detected during a specific totaling period having a specific relation to the waveform of the AC current. The total value as the value to be judged (referred to as xe2x80x9cjudgment valuexe2x80x9d later) is compared with a setting value and it is judged that the defect exists in the container when the total value exceeds the setting value.
The aforementioned totaling period is required to be the specific period always having the specific relation to the waveform of the AC current. There may be selected a period which can be detected on a easily detectable zero cross or peak point, such as period of positive half cycle, a period of negative half cycle, a period of xc2xc cycle from the zero cross point to the peak point, a period of xc2xc cycle from the peak point to the zero cross point, and a period of n cycle (n is an integer of one or more) for the specific period.
As there is provided the A/D converter, which converts the instantaneous value of the detection signal of the AC current flowing through the container and the contained liquid and also as there is determined the totaling period of the specific period having the predetermined relation to the waveform of the AC current so that the total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current detected during the totaling period is obtained as aforementioned, variations in the instantaneous value of the AC current due to the existence of the defect in the container are totaled during the totaling period. Thus, even though there is just a small difference between the crest value of the waveform of the AC current obtained when the container has the defect and the crest value of the AC current obtained when the container has no defect, the total value can have a larger difference between them.
Thus, according to the aforementioned method, even though there is generated no large difference between the crest value of the AC current flowing through electrodes when the container has the defect causing the leakage of the liquid of the non-electrolyte sealed therein and that when it has no defect, with the container containing the non-electrolyte, the existence of the defect causing the leakage can be inspected with the high accuracy.
Furthermore, as the total value obtained by successively adding the digital values corresponding to the absolute values of the instantaneous values of the detected AC current during the fixed period as aforementioned is used as the judgment value, a noise which would momentarily overlaps the AC current provides little influence on the judgment value whereby the S/N ratio of the judgment value (the signal to be used for the judgment) is made higher. Thus, the misdetection is prevented and the existence of the defect can be judged with the high precision.
Furthermore, according to the aforementioned method, even though there is an insignificant difference between the crest value of the AC voltage waveform obtained when the container has the defect and the crest value of the AC voltage waveform obtained when the container has no defect, the total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current has the larger difference generated between them so that the existence of the defect can be positively judged. Thus, in the case where the liquid in the container is the electrolyte, the existence of the defect can be positively judged even though the crest value of the AC inspection voltage is set up at the low value to the degree in which the discharge phenomenon arising at the defect portion is kept in the fore-discharge phenomenon. Therefore, the existence of the leakage can be inspected without possible deterioration of the container by making the crest value of the AC inspection voltage lower than that of the prior art.
In the preferred embodiment, there is performed two or more times a total value arithmetical operation process in which there is arithmetically operated the total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current detected from the A/D converter during the specific totaling period having the predetermined relation to the waveform of the AC current. The maximum value of the total values obtained in the respective total value arithmetical operation processes performed two or more times is determined as the judgment value (the value to be judged) and it is judged that the defect exists in the container when the thus determined judgment value exceeds the setting value in the comparison therewith.
As the maximum value of the total values obtained in the respective total value arithmetical operation processes performed two or more times is determined as the judgment value in this manner, the probability of obtaining the information including the existence of the defect can be more improved than that in the case where the total value arithmetical operation process is performed only one time and therefore, the precision of the inspection can be improved.
In the case where the electrodes contact for a short time the containers conveyed by a conveyor in the step of manufacturing the products without stopping conveying the containers, as the maximum value of the total values obtained in the respective total value arithmetical operation processes performed two or more times is used as the judgment value as aforementioned, the probability of obtaining the information including the existence of the defect can be heightened and the precision of the inspection can be improved.
In another preferred embodiment of the invention, there is performed two or more times a total value arithmetical operation process in which there is arithmetically operated the total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current detected from the A/D converter during the specific totaling period having the predetermined relation to the waveform of the AC current, the average value of the total values obtained in the respective total value arithmetical operation processes performed two or more times is determined as the judgment value (the value to be judged) and it is judged that the defect exists in the container when the thus determined judgment value exceeds the setting value in the comparison therewith.
Also in the case where the average value of the total values obtained in the respective total value arithmetical operation processes performed two or more times is determined as the judgment value as aforementioned, the probability of obtaining the information including the existence of the defect can be more improved than that in the case where the total value arithmetical operation process is performed only one time and therefore, the precision of the inspection can be improved.
Moreover, as the average value of the total values obtained two or more times is determined as the judgment value in this manner, the ratio of S/N of the judgment value can be more improved than that in the case where the total value arithmetical operation process is performed only one time.
The leakage inspection apparatus of the present invention is for an insulating container that seals a liquid and relates to the apparatus for inspecting whether the defect causing the leakage of the container exists.
In the invention, there are provided an inspection power source unit to apply an inspection voltage of high AC voltage across the container, a current detector to detect an AC current flowing from the inspection power source unit through the container and the liquid in the container, an A/D converter to digitally convert an instantaneous value of the detected AC current, judgment value arithmetical operation means to arithmetically operate as the judgment value the total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current detected during a totaling period having a fixed relation to the waveform of the AC current and judgment means to compare the judgment value with a setting value to judge that the container has a defect when the judgment value exceeds the setting value.
The aforementioned judgment value arithmetical operation means may be so constructed that there is performed two or more times a total value arithmetical operation processes in which there is arithmetically operated the total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current detected from the A/D converter during the specific totaling period having a fixed relation to the waveform of the detected AC current and the maximum value of the total values obtained in the respective total value arithmetical operation processes performed two or more times is determined as the judgment value.
Also, the aforementioned judgment value arithmetical operation means may be so constructed that there is performed two or more times a total value arithmetical operation processes in which there is arithmetically operated the total value of the digital values corresponding to the absolute values of the instantaneous values of the AC current detected from the A/D converter during the specific totaling period having a fixed relation to the waveform of the detected AC current and the average value of the total values obtained in the respective total value arithmetical operation processes performed two or more times is determined as the judgment value.
It is preferable that a period such as a period of xc2xd cycle of the waveform of the AC current and a period of n cycle (n is an integer of one or more) is selected as the aforementioned totaling period.